The objective of this study is to determine the role played by light induced free radicals in chemically induced skin photosensitivity. Light irradiation of 4-aminobenzenesulfonamide (sulfanilamide) resulted in the production of the following radicals: NH2, 4-NHC6H4SO2NH2, H., 4-H2NO2SC6H4, SO2NH2, 4-NH2C6H4SO2, SO3-. Which radicals were produced depended on the pH of the solution and the wavelength of irradiating light. Under the same conditions, sulfacetamide, sulfadiazine, sulfathiazole, carbutamide, and tolbutamide each gave rise to several of the same radicals, notably SO3-. The generation of these free radical species may explain the phototoxic and photoallergic properties of these compounds.